The achievement of laminar flow over the surface of an aircraft can lead to significant drag reduction and hence fuel savings. It is known to delay the transition from laminar to turbulent flow over a surface of an aircraft by applying suction to the surface. The boundary layer is sucked through pores in the surface to prevent the onset of turbulence. This is known as laminar flow control.
The ability of the surface to achieve and maintain laminar flow is however impaired by contamination. The primary contaminants on an aircraft surface are ice and insect debris entrained in the air flow. The build up of ice and insect debris on the surface can cause the premature transition of the air flow from laminar to turbulent flow and can block the pores in the surface through which the boundary layer of air is sucked.
Insect contamination is eliminated by extruding a protective liquid from the porous surface. The liquid washes off any insects already attached and wets the outer surface so that further insects will not adhere. The liquid is only extruded from a limited part of the surface as the airflow passing over the surface carries it rearwards to wash the rest of the surface.
During flight icing of the aircraft surfaces can occur. Protection against ice formation is required since icing for example at the leading edge of an engine nacelle can restrict the airflow to the engine. In addition damage may result if ice is allowed to build up on an aircraft surface and then breaks away.
Ice protection systems are used to protect aircraft surfaces against ice formation. The ice protection systems operate to heat the aircraft surfaces where ice is likely to form. Basic systems of ice protection use hot air, electrical heating or a combination of electrical power and hot air.
Problems arise in using ice protection systems on surfaces having laminar flow control and contamination protection. EP0599502-A1 describes a porous surface which provides both laminar flow control by suction and which extrudes liquid to prevent insect contamination. Heating of the porous surface is inhibited by the double skin construction of the surface. Further the porous backing sheet is plastic and cannot withstand heating by the ice protection system.